The invention relates to a data storage medium having an optical information carrier which comprises a spiral-wound polymer film.
DE-298 16 802 describes a data storage medium having an information carrier, wound in a plurality of plies onto a winding core in a spiral fashion, for optically readable information units. The information carrier may comprise a polymer film, with an adhesion layer being located between each pair of adjacent plies. Information can be written to this data storage medium by locally heating the polymer film by means of a write beam of a data drive, as a result of which the refractive index and thus the reflecting power (reflectivity) change locally at the interface of the polymer film. This may be detected by means of a read beam in the data drive. By focussing the write beam or read beam, information may be specifically written to or read from a preselected ply of the information carrier. The winding core may be optically transparent and may have a recess in its central area that serves to accommodate the read/write device of a data drive. The read/write device is moved relative to the data storage medium, while the data storage medium is stationary, so that the data storage medium need not be balanced to take account of a rapid rotational motion.
In the existing data storage medium, the winding core is a disruptive factor, since its optical quality is inadequate unless it is manufactured with a high degree of elaborateness. Since, when the data storage medium is used in a data drive whose read/write device is situated in the recess of the winding core, the winding core is required to transmit a beam a number of times during each read operation, inadequate optical quality has particularly unfavorable consequences.
It is an object of the invention to improve the existing data storage medium such that no problems arise as a consequence of inadequate optical quality of a winding core and yet the data storage medium can be manufactured economically.
This object is achieved by means of a data storage medium having the features of claim 1. Claim 7 specifies a process for producing a data storage medium of this kind. Claim 9 relates to the use of such a data storage medium in a drive that is attuned to it. Advantageous embodiments of the invention follow from the dependent claims.
The data storage medium of the invention has an optical information carrier which comprises a spiral-wound polymer film. The central area of the data storage medium is provided with a recess whose periphery is formed by the innermost winding of the polymer film. Accordingly, the data storage medium contains no separate winding core such as is the case with the existing data storage medium.
The reading of information or data from the data storage medium of the invention and—if the data storage medium is set up for the input of data by the user—the writing of information to the data storage medium is therefore not hindered by a winding core. Accordingly, the data storage medium of the invention is particularly suitable for use in a drive in which a read device and an optional write device are arranged in the recess in the central area of the data storage medium.
Preferably, the polymer film is wound in a plurality of polymer film plies through which information can be read from a preselected polymer film ply and, optionally, can be written to a preselected polymer film ply. There may be an adhesion layer between each pair of adjacent polymer film plies in order to fix the polymer film plies to one another. For example, from 10 to 30 polymer film plies may be wound atop one another, or else a greater or lesser number. At a polymer film thickness of between 10 μm and 100 μm, preferably below 50 μm or around 35 μm, the information on different polymer film plies can be separated from one another with good resolution by means, for example, of read/write devices which are known from DVD technology. An adhesion layer may, for example, have a thickness in the range between 1 μm and 40 μm, preferably below 25 μA or around 2 μm. A suitable adhesion agent comprises, for example, an acrylate adhesive which is free from air bubbles and which is crosslinked, for example, chemically or by irradiation with UV or electron beams. With a multi-ply data storage medium construction of this kind, it is possible to achieve a very high storage density. Furthermore, even without a winding core, the mechanical stability is sufficient and may be increased further, for example, by inserting the data storage medium into an outer sleeve. Slight deviations of the wound polymer film from an ideal spiral form, such as might arise, for example, in the absence of a winding core by deformation of the windings of the polymer film, are not disruptive to the reading and/or writing of data; since the focus of a read beam and/or of a write beam may be tracked without any problems in order to remain in a preselected ply of the information carrier. If the read device and the optional write device are disposed in the recess in the central area of the data storage medium and are moved relative to the data storage medium in order to read and/or write information, while the data storage medium is stationary, even any possible imbalance in the data storage medium is irrelevant.
Preferably, the refractive index of the adhesion layer differs only slightly from the refractive index of the polymer film, in order to minimize disruptive reflections of the read beam or of the write beam at a boundary between a polymer film ply and an adjacent adhesion layer. It is particularly advantageous if the difference in the refractive indices is less than 0.005. Any difference in the refractive indices may, however, be utilized for the purpose of formatting the data storage medium.
In one preferred embodiment of the data storage medium of the invention, the refractive index of the polymer film can be changed locally by heating. Suitable material for the polymer film comprises, for example, polymethyl methacrylate (PMMA) or biaxially oriented polypropylene (BOPP). If polypropylene, following extrusion to the film, is pretensioned in two planes, a high inherent energy is stored in the material. In the case of local heating, by means of a write beam, for example, there is a severe change in the material by reformation, and this is so even when a relatively small amount of energy is deposited per unit area. In this way it is possible, for example, to achieve a change in refractive index of approximately 0.2 over an area for one stored information unit with a diameter or side length of approximately 1 μm, and this is readily detectable by means of a read beam.
The polymer film may be assigned an absorber which is set up at least partly to absorb a write beam and to emit the generated heat at least partly, locally, to the polymer film. The absorber comprises, for example, dye molecules which are present in the polymer film or in an adhesion layer adjacent to the polymer film, and permits local heating of the polymer film, sufficient to change the refractive index, for a relatively low write beam intensity.
The data storage medium of the invention may be produced by winding the polymer film spirally onto a winding body and subsequently withdrawing the winding body from the central area of the data storage medium. If there is to be an adhesion layer between each pair of adjacent polymer film plies, then in one advantageous embodiment of the process the polymer film is provided on one side with an adhesion layer which faces outward when the polymer film is wound onto the winding body. The adhesion layer is therefore unable to bond to the winding body, so that at the end of the winding operation the winding body can be withdrawn without any problems from the central area of the data storage medium. When the winding operation has been concluded, the outward-facing adhesion layer on the outermost polymer film ply may be covered with an additional, nonadhering material ply or else, for example, may be removed by means of a solvent or rendered nonadhesive by chemical or physical treatment. In this way, a data storage medium of the invention can be produced economically.